Image sensor illuminated and connected on its back side

ABSTRACT

An image sensor including a semiconductor layer; a stack of insulating layers resting on the back side of the semiconductor layer; a conductive layer portion extending along part of the height of the stack and flush with the exposed surface of the stack; laterally-insulated conductive fingers extending through the semiconductor layer from its front side and penetrating into said layer portion; laterally-insulated conductive walls separating pixel areas, these walls extending through the semiconductor layer from its front side and having a lower height than the fingers; and an interconnection structure resting on the front side of the semiconductor layer and including vias in contact with the fingers.

This application claims the priority benefit of French Patentapplication number 14/62456, filed on Dec. 15, 2014, the contents ofwhich is hereby incorporated by reference in its entirety to the maximumextent allowable by law.

BACKGROUND

1. Technical Field

The present disclosure relates to an image sensor and to a method formanufacturing the same.

2. Description of the Related Art

Image sensors comprising a semiconductor layer having a first side,called back side, intended to receive an illumination, and having asecond side, called front side, coated with an interconnectionstructure, are known. Components such as transistors are also currentlyformed on the front side. Such image sensors where insulated connectionsrun from the interconnection structure up to the back side through thesemiconductor layer are here considered. Such insulated connections aregenerally formed in openings which are wide as compared with the widthof insulating walls formed between pixel areas of the image sensor, theinsulating walls being formed from the front side and the wide openingsbeing formed from the back side.

This type of image sensor has various disadvantages, some of which aredesired to be overcome herein.

BRIEF SUMMARY

Thus, an embodiment comprises an image sensor comprising a semiconductorlayer; a stack of insulating layers resting on the back side of thesemiconductor layer; a conductive layer portion extending along part ofthe height of the stack and flush with the exposed surface of the stack;laterally-insulated conductive fingers extending through thesemiconductor layer from its front side and penetrating into said layerportion; laterally-insulated conductive walls separating pixel areas,the walls extending through the semiconductor layer from its front sideand having a lower height than the fingers; and an interconnectionstructure resting on the front side of the semiconductor layer andcomprising vias in contact with the fingers.

According to an embodiment, the semiconductor layer is made of silicon.

According to an embodiment, the stack successively comprises a firstsilicon oxide layer resting on the back side, a silicon nitride layer,and a second silicon oxide layer.

According to an embodiment, the walls cross the first oxide layer andpenetrate into the silicon nitride layer, the fingers cross the firstoxide layer and the silicon nitride layer, and said layer portionextends along the entire height of the second oxide layer.

According to an embodiment, the walls and the fingers are made of dopedpolysilicon lined with an insulating layer.

According to an embodiment, the width of the walls and of the fingers issmaller than 0.5 μm.

An embodiment provides a method of manufacturing an image sensorcomprising the successive steps of:

a) forming a stack of insulating layers on the back side of asemiconductor layer;

b) simultaneously etching from the front side of the semiconductor layerfirst trenches and second trenches shallower than the first ones, thefirst trenches penetrating into said stack, the second trenchesseparating pixel areas of the image sensor;

c) forming an insulating layer on the walls of the first and secondtrenches;

d) filling the first and second trenches with a first conductivematerial;

e) on the front side of the semiconductor layer, forming aninterconnection structure comprising vias in contact with the firstconductive material filling each first trench;

f) etching a cavity in the stack to expose the first conductive materialfilling each of the first trenches; and

g) filling said cavity with a second conductive material.

According to an embodiment, at step b), a masking layer comprising firstand second openings is formed on the front side, the first openingsbeing wider than the second openings.

According to an embodiment, said stack successively comprises a firstsilicon oxide layer resting on the back side, a silicon nitride layer,and a second silicon oxide layer and, at step b), the first trenches areetched all the way into the second silicon oxide layer, the nitridelayer being used as an etch stop layer at step f).

The foregoing and other features and advantages will be discussed indetail in the following non-limiting description of specific embodimentsin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-section view schematically showing an example of animage sensor;

FIG. 2 is a cross-section view schematically showing an embodiment of animage sensor; and

FIGS. 3A to 3E are simplified cross-section views illustratingsuccessive steps of a method of manufacturing the image sensor of FIG.2.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the various drawings and, further, the various drawings are not toscale.

In the following description, when reference is made of terms qualifyingthe position and orientation such as “left-hand”, “right-hand”,“bottom”, “above”, “under”, “upper”, “lower”, etc., reference is made tothe representation of the concerned elements in the drawings to whichreference is made.

FIG. 1 is a cross-section view schematically showing an example of animage sensor.

The image sensor comprises a P-type doped semiconductor layer 1 havingits back side F1 intended to receive an illumination and coated withinsulating layers 3, and having its front side F2 coated with aninterconnection structure 5. The assembly of semiconductor layer 1, ofinsulating layers 3, and of the interconnection structure is mounted ona support 7, the interconnection structure extending between support 7and semiconductor layer 1. Interconnection structure 5, only a portionof which is shown, is formed from portions of metal layers 9 separatedby insulating layers 11 crossed by vias 13.

On the right-hand side of FIG. 1, semiconductor layer 1 comprisesphotodiodes 15 and components such as transistors formed on front sideF2. Two photodiodes 15 corresponding to pixels and to gate 17 of atransistor are shown. Each photodiode comprises an N-type doped layer 19formed in semiconductor layer 1. Insulating layers 3 are used asantireflection layers and are coated, in front of each photodiode 15,with a color filter 21 topped with a lens 23. Walls 25 made ofsemiconductor material 27 lined with an insulating layer 29 extendthrough semiconductor layer 1 from its front side F2 and separate thephotodiodes from one another. At the level of front side F2, the imagesensor components and conductive material 27 of walls 25 are in contactwith vias 13 of interconnection structure 5.

On the left-hand side of FIG. 1, an insulated connection 31 enables tocreate a contact between an element of interconnection structure 5 and apad, not shown, formed on the back side of the image sensor. Insulatedconnection 31 is formed from an opening 33 having its lateral wallscoated with an insulating layer 35, and with a metal layer 37 coatinginsulating layer 35 and the bottom of opening 33. Opening 33 is widerthan walls 25 for example, 100 times wider. Opening 33 extends from theexposed surface of stack 3 of insulating layers all the way to frontside F2 of semiconductor layer 1, and crosses a silicon oxide layer 39formed in semiconductor layer 1 on its front side F2. At the bottom ofopening 33, metal layer 37 is in contact with a doped polysilicon layer41 which is connected by vias 13 to an element of the interconnectionstructure.

In practice, this image sensor comprises many insulated connections 31and, in top view, not shown, insulated connections 31 are arrangedaround a central area of the image sensor having photodiodes 15 and theassociated components formed therein.

To manufacture the image sensor of FIG. 1, walls 25 are formed fromfront side F2 of semiconductor layer 1, after which the interconnectionstructure is formed on side F2. The assembly of semiconductor layer 1and of interconnection structure 5 is then mounted on support 7 as shownin FIG. 1. Then, only openings 33 are formed by reactive ion etchingfrom the exposed surface of stack 3, or back side of the sensor, andinsulated connections 31 are formed in openings 33.

This manufacturing method implies providing a succession of stepscarried out from the back side of the sensor to form openings 33 andinsulated connections 31 running from this back side to front side F2 ofthe semiconductor layer. This complicates the sensor manufacturingmethod and brings about several disadvantages. Due to the fact that theopenings are formed from the back side of the sensor and that they havea significant width, currently a side length or a diameter greater than50 μm, it is difficult to obtain a sensor having a planar back side,which complicates the forming of filters 21 and of lenses 23 on thisside. The forming of openings 33 by reactive ion etching introducesfixed charges into semiconductor layer 1 and into components of thesensor, which disturbs its proper operation. Further, on etching ofopenings 33, polysilicon layer 41 may be overetched, which may degradethe quality of the electric connection between interconnection structure5 and insulated connection 31.

FIG. 2 is a cross-section view showing an embodiment of an image sensor.

The image sensor comprises, on a support 7, an interconnection structure5, a P-type doped semiconductor layer 1, and a stack 3 of insulatinglayers. Elements 1, 3, 5, and 7 are arranged relative to one another inthe same way as in FIG. 1. More particularly, in this example, stack 3successively comprises insulating layers 51, 53, and 55, layer 51resting on back side F1 of semiconductor layer 1.

On the right-hand side of FIG. 2, the sensor comprises photodiodes 15,components such as transistors, walls 25 separating the photodiodes, andfilters 21 resting on stack 3 opposite the photodiodes, each filterbeing topped with a lens 23. As previously described, walls 25 are madeof a conductive material 27 lined with an insulating layer 29 andextending through semiconductor layer 1 from its front side F2. In thisexample, walls 25 penetrate into insulating layer 53 of stack 3. At thelevel of front side F2, image sensor components and walls 25 areconnected to vias 13 of interconnection structure 5.

On the left-hand side of FIG. 2, fingers 57 made of conductive material27 laterally lined with an insulating layer 29 enable to create acontact between an element of interconnection structure 5 and a metalpad 59 formed on the back side of the sensor. Each finger 57 extendsfrom front side F2 of semiconductor layer 1 and penetrates into pad 59,and the portion of the finger penetrating into the pad is not lined withinsulating layer 29. Metal pad 59 is flush with the back side of thesensor and, in this example, extends along the entire height ofinsulating layer 55. On front side F2, semiconductor layer 1 maycomprise a shallow recess filled with an insulator 61 lining a lowerportion of each finger 57. At the level of front side F2, fingers 57 areconnected to an element of interconnection structure 5 by vias 13. Thus,fingers 57 are used as an insulated connection between pad 59 formed onthe back side of the sensor and vias 13 of the interconnection structurearranged on front side F2 of semiconductor layer 1.

FIGS. 3A to 3E are simplified cross-section views illustratingsuccessive steps of an embodiment of the image sensor of FIG. 2.

FIG. 3A shows a structure successively comprising a semiconductor layer1, a stack 3 of successive insulating layers 51, 53, and 55, and ahandle or a support 63 such as a silicon wafer, insulating layer 51resting on a side F1 of semiconductor layer 1. A masking layer 65 hasbeen deposited on side F2 of semiconductor layer 1 opposite to its sideF1. Masking layer 65 comprises openings 67 and 69 at the locations wherefingers and walls are desired to be formed, respectively. In thisexample, the semiconductor layer comprises a shallow recess filled withan insulator 61 formed on side F2 above openings 67. Trenches 71 and 73are simultaneously formed through semiconductor layer 1 all the way intostack 3 of insulating layers 51, 53, and 55 by etching from openings 67and from openings 69 respectively. Openings 67 are selected to be widerthan openings 69 so that trenches 71 are deeper than trenches 73. Inthis example, trenches 71 cross insulating layers 51 and 53 andpenetrate into insulating layer 55, and trenches 73 cross insulatinglayer 51 and penetrate into insulating layer 53.

Semiconductor layer 1 may be made of silicon, germanium, or of any othersemiconductor material capable of forming an image sensor. In thisexample, layer 1 is P-type doped. Stack 3 may be formed of a siliconoxide layer 51 formed by thermal oxidation, of a silicon nitride layer53, and of a deposited silicon oxide layer 55. The width of trenches 71and 73 is for example smaller than 2 μm, preferably smaller than 0.5 μm,and may be equal to 0.35 μm and 0.2 μm, respectively. The thickness ofsemiconductor layer 1 may be in the range from 2 to 5 μm, and is forexample equal to 3 μm. The thickness of thermal oxide layer 51 may be inthe range from 5 to 20 nm, and is for example equal to 7.5 nm. Thethickness of silicon nitride layer 53 may be in the range from 30 to 80nm, and is for example equal to 55 nm. The thickness of deposited oxidelayer 55 may be in the range from 80 to 200 nm, and is for example equalto 100 nm.

FIG. 3B shows the structure of FIG. 3A after the deposition of aninsulating layer 29 which covers the walls of trenches 71 and 73,followed by the deposition of a layer of a conductive material 27 whichfills trenches 71 and 73. Fingers 57 of an insulated connection andinsulated conductive wall 25 are thus formed. Material 27, insulatinglayer 29, and masking layer 65 have been removed from surface F2. Beforeor after the forming of walls 25 and of fingers 57, the image sensorcomponents are formed in semiconductor layer 1 from its surface F2.These components for example include photodiodes 15 each comprising anN-type doped layer 19 extending between walls 25 and transistors, a gate17 thereof being schematized.

As an example, conductive material 27 may be heavily-doped polysilicon,for example, of type P, where the dopant atom concentration may begreater than 10¹⁹ at·cm⁻³. Insulating layer 29 may be formed of asilicon oxide layer coated with a silicon nitride layer used as adiffusion barrier. The thickness of insulating layer 29 may be in therange from 5 to 30 nm, and is for example equal to 25 nm.

FIG. 3C shows the structure of FIG. 3B after the forming of aninterconnection structure 5 on side F2 of semiconductor layer 1 bysuccessive steps of depositing and etching insulating layers 11 andmetal layers. These steps are carried out so that interconnectionstructure 5 comprises elements, for example, vias 13, connected tocomponents of the image sensor and to conductive material 27 of walls 25and of fingers 57.

FIG. 3D shows the structure of FIG. 3C flipped and fixed to a handle ora support 7 such as a silicon wafer, interconnection structure 5extending between support 7 and semiconductor layer 1. Support 63 hasbeen removed. Above fingers 57, a cavity 79 has been etched ininsulating stack 3 so that a portion of conductive material 27 of eachfinger 57 protrudes above the bottom of cavity 79. In this example,insulating layer 53 of stack 3 is used as an etch stop layer and cavity79 extends along the entire height of insulating layer 55.

FIG. 3E shows the structure of FIG. 3D after forming in cavity 79 aconductive pad 59 for example comprising a tantalum and/or tantalumnitride layer topped with an aluminum layer. Pad 59 is flush with theexposed surface of stack 3 and the sensor has a planar back side. Atnext steps, not shown, an assembly of a color filter 21 topped with alens 23 is formed on the back side of the sensor, opposite eachphotodiode.

As previously described in relation with FIGS. 3A to 3E, in the methodof manufacturing an image sensor of the type in FIG. 2, insulatedconductive walls 25 and insulated connections formed from fingers 57 aresimultaneously formed from the same front side F2 of a semiconductorlayer 1.

This simultaneous forming of insulated conductive walls 25 and offingers 57 of connection from the front side of the sensor has manyadvantages. First, it results in a decrease of the number ofmanufacturing steps of an image sensor of the type in FIG. 2 as comparedwith a sensor of the type in FIG. 1. Another advantage is that the backside of the sensor is planar and thus that the steps of forming colorfilters 21 and lenses 23 on this surface are simpler to implement thanin a sensor of the type in FIG. 1.

Further, to compare an insulated connection of the type in FIG. 1 withan insulated connection of the type in FIG. 2, tests carried out by theinventors have shown that an insulated connection of the type in FIG. 2has a total access resistance smaller than that of an insulatedconnection of the type in FIG. 2. This may particularly be imputed tothe fact that the contact resistance between conductive fingers 57 of aninsulated connection of the type in FIG. 2 and vias 13 of aninterconnection structure 5 is particularly low.

Specific embodiments have been described. Various alterations,modifications, and improvements will readily occur to those skilled inthe art. In particular, it should be understood that fingers 57 may beused as an insulated connection between two opposite sides of asemiconductor layer in the case where a device other than an imagesensor is formed in this semiconductor layer, for example, a devicecomprising no photodiodes.

The dimensions, the conductivity types, and the materials of the variousregions previously indicated as an example may be adapted by thoseskilled in the art. For example, semiconductor layer 1 may be N-typedoped, layer 19 of each photodiode 15 then P-type doped. A stack 3 ofinsulating layers comprising other layers than those previouslyindicated may also be provided.

Steps of the manufacturing method may be modified, added or suppressedand the order in which these steps are carried out may be adapted bythose skilled in the art. In particular, the width of openings 69 and 71may be adapted according to the depth of trenches 71 and 73 which aredesired to be formed and these trenches may penetrate into stack 3 ofinsulating layers all the way to levels different from those previouslyindicated. It is possible to form no insulator 61 in semiconductor layer1.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1. An image sensor comprising: a semiconductor layer; a stack ofinsulating layers resting on a back side of the semiconductor layer; aconductive layer portion extending along part of a height of the stackand flush with an exposed surface of the stack; laterally-insulatedconductive fingers extending through the semiconductor layer from afront side of the semiconductor layer and penetrating into saidconductive layer portion; laterally-insulated conductive wallsseparating pixel areas, the walls extending through the semiconductorlayer from the front side and having a lower height than the fingers;and an interconnection structure resting on the front side of thesemiconductor layer and including vias in contact with the fingers. 2.The image sensor of claim 1, wherein the semiconductor layer is made ofsilicon.
 3. The image sensor of claim 2, wherein the insulating layersof the stack successively include a first silicon oxide layer resting onthe back side, a silicon nitride layer, and a second silicon oxidelayer.
 4. The sensor of claim 3, wherein the walls cross the first oxidelayer and penetrate into the silicon nitride layer, the fingers crossthe first oxide layer and the silicon nitride layer, and said conductivelayer portion extends through an entire height of the second oxidelayer.
 5. The sensor of claim 1, wherein the walls and the fingers aremade of doped polysilicon lined with an insulating layer.
 6. The sensorof claim 1, wherein the width of the walls and of the fingers is smallerthan 0.5 μm.
 7. A method, comprising: manufacturing an image sensor, themanufacturing including: forming a stack of insulating layers on a backside of a semiconductor layer; forming a conductive layer portionextending along part of a height of the stack and flush with an exposedsurface of the stack; forming laterally-insulated conductive fingersextending through the semiconductor layer from a front side of thesemiconductor layer and penetrating into said conductive layer portion;forming laterally-insulated conductive walls separating pixel areas, thewalls extending through the semiconductor layer from the front side andhaving a lower height than the fingers; and forming an interconnectionstructure resting on the front side of the semiconductor layer andincluding vias in contact with the fingers.
 8. The method of claim 7,wherein forming the laterally insulated conductive fingers includes:etching from the front side of the semiconductor layer first trenchespenetrating into said stack; forming an insulating layer on walls of thefirst trenches; and filling the first trenches with a first conductivematerial.
 9. The method of claim 8, wherein forming laterally-insulatedconductive walls includes: simultaneously with etching the firsttrenches, etching from the front side of the semiconductor layer secondtrenches shallower than the first trenches; forming the insulating layeron walls of the second trenches; and filling the second trenches withthe first conductive material.
 10. The method of claim 9, wherein: onthe front side of the semiconductor layer, forming an interconnectionstructure comprising vias in contact with the first conductive materialfilling each first trench forming the laterally insulated conductivefingers includes: etching a cavity in the stack to expose the firstconductive material filling each of the first trenches; and filling saidcavity with a second conductive material.
 11. The method of claim 10,wherein; forming said stack comprises successively forming a firstsilicon oxide layer on the back side, forming a silicon nitride layer onthe first silicon oxide layer, and forming a second silicon oxide layeron the silicon nitride layer; etching the first trenches includesetching the first trenches into the second silicon oxide layer; andetching the cavity includes etching the cavity while using the nitridelayer as an etch stop layer.
 12. The method of claim 9, furthercomprising forming a masking layer, having first and second openings, onthe front side of the semiconductor layer prior to etching the first andsecond trenches, the first openings being wider than the secondopenings.